Aluminum silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys, while alloys containing more than 11.6% silicon are referred to as hypereutectic alloys. The solidification range, which is a temperature range over which the alloy will solidify, is the range between the liquidus temperature and the invariant eutectic temperature. The wider or greater the solidification range, the longer it will take an alloy to solidify at a given rate of cooling.
Hypoeutectic aluminum silicon alloys, those containing less than 1.16% silicon, have seen use for many years. The unmodified alloys have a microstructure consisting of primary aluminum dendrites with a eutectic composed of acicular silicon in an aluminum matrix.
On the other hand, hypereutectic aluminum-silicon alloys, those containing more than 11.6% silicon, contain primary silicon crystals which are precipitated as the alloy is cooled from solution temperature. Due to large precipitated primary silicon crystals, these alloys have good wear resistant properties, but the hypereutectic aluminum-silicon alloys are difficult to machine, a condition which limits their use as casting alloys. While alloys of this type have good fluidity, they have a large or wide solidification range, and the solidification range will increase dramatically as the silicon content is increased.
Normally a solid phase in a "liquid plus solid" field, has either a lower or higher density than the liquid phase, but almost never the same density. If the solid phase is less dense than the liquid phase, floatation of the solid phase will result. On the other hand, if the solid phase is more dense, a settling of the solid phase will occur. In either case, an increased or widened solidification range will increase the time period for solidification and accentuate the phase separation. With a hypereutectic aluminum-silicon alloy, the silicon particles have a lesser density than the liquid phase, so that the floatation condition prevails, and the alloy solidifies with a large mushy zone, because of its high thermal conductivity, and the absence of skin formation typical of steel castings. Thus, as the solidification range is widened, the tendency for floatation of large primary silicon particles increases, thus resulting in a less uniform distribution of silicon particles in the cast alloy.
A wide solidification range can also result in significant amounts of microporosity, because the wide mushy zone does not permit good feeding of the liquid aluminum phase as it solidifies and shrinks about 6.9% in volume. When the cast alloy is used as an engine block, the microporosity results in high oil consumption in a four-stroke engine.
Hypereutectic aluminum-silicon alloys containing precipitated primary silicon crystals have had commercial applicability only because of the refinement of the primary silicon phase by phosphorus additions to the melt, as disclosed in U.S. Pat. No. 1,387,900. The addition of small amounts of phosphorous causes a precipitation of aluminum-phosphorous particles, which serve as the active nucleant for the primary silicon phase. Due to the phosphorous refinement, the primary silicon particles are of smaller size and have a more uniform distribution, so that the alloys can be used in applications requiring the manufacturing attribute of machinability and the engineering attribute of wear resistance. However, phosphorous refined alloys of this type do not have any significant level of ductility and thus are not used in more diverse engineering applications, requiring machinability, wear resistance, and ductility.
Hypoeutectic aluminum-silicon alloys, those containing less than 11.6% silicon, are relatively non-ductile or brittle because of the large irregular shape of the acicular eutectic silicon phase. It has been recognized that the growth of the eutectic silicon phase can be modified by the addition of small amounts of sodium or strontium, thereby increasing the ductility of the hypoeutectic alloy.
Therefore, while it is known that the primary silicon phase in a hypereutectic aluminum silicon alloy can be refined by the addition of phosphorous and it is further known that the eutectic silicon phase in a hypoeutectic aluminum silicon alloy can be modified with sodium or strontium, it is not possible to include both the additions of phosphorous and sodium or strontium in a hypereutectic alloy, since sodium and strontium neutralize the phosphorous effect. Thus, there has been no commercially available hypereutectic aluminum-silicon alloy with both a refined primary silicon phase and a modified eutectic silicon phase.